This was actually working anyways, because NULL pointers don't trap,
and the pointed-to char is currently 0x98, not a digit.

But I prefer to fix it before we do one of the following:
  - move to a new cpu that traps on null
  - use RAM at a different address than zeero
  - use "mvi r0, 0" instead of "xor r0,r0,r0" (would parse as 4).
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>